A familiar cord which is used with telephone station equipment is a retractile one and is made of cordage which is wound helically about a mandrel. The wound cordage is then subjected to a heat-treating temperature after which it is removed from the cord while the helical direction of wind is reversed. See, for example, U.S. Pat. Nos. 2,920,351 and 3,024,497 issed on Jan. 12, 1960 and on Mar. 13, 1962, respectively, in the names of E. C. Hardesty and D. L. Myers and U.S. Pat. No. 3,988,092, all incorporated by reference hereinto.
Retractile cords used on telephones to connect a handset to a base must have sufficient retractility to insure that they will return in a controlled gradual manner to their normal retracted form after having been extended and then released. However, such cords which are commonly known as spring cords must not be so strongly retractile that they require excessive forces to extend them. If a spring cord is too unyielding, the instrument to which it is connected may be removed on or pulled from its support. To prevent this, spring cords that are connected to lightweight desk-type or bedroom-type telephone handsets must be readily extensible.
While excessive retractility must be avoided, a cord must not be made so extensible that its distended helix fails to return to a retracted condition following use of the telephone. This is especially important in order to prevent unsightly, excessive sag of retractile cords which are used on wall-mounted telephones. Further, it is desirable that the retracted length of the spring cord be as short as possible.
Spring cords of the type used on telephones are generally constructed of cordage having a plurality of individually insulated, mandrelated flexible conductors comprising tinsel ribbons. In the past, each conductor was covered with a nylon knit and then insulated with a polyvinyl chloride (PVC) composition. Subsequently, the plurality of individually insulated conductors were jacketed with a PVC composition and had a circular cross-section. See, for example, U.S. Pat. No. 3,037,068 issued May 29, 1962 in the name of H. L. Wessel, incorporated by reference hereinto.
A new modular concept in telephone cord design includes the use of modular plugs for terminating the cord conductors. Jacks adapted to receive the plugs are mounted in the telephone housing or base and in a wall terminal thereby permitting easy replacement of either the line or spring cord by a customer or an installer. See, for example, U.S. Pat. Nos. 3,699,498 and 3,761,869 issued Oct. 17, 1972 and Sept. 25, 1973, respectively, in the names of E. C. Hardesty, C. L. Krumreich, A. E. Mulbarger, Jr. and S. W. Walden, and U.S. Pat. No. 3,860,316 issued Jan. 14, 1975 in the name of E. C. Hardesty, all incorporated by reference hereinto.
Conversion of modularity with its associated plug-terminated cordage necessitated the development of telephone cordage having a smaller cross-section than that used in the past. A cordage design suitable for use with modular plugs incorporated smaller conductors arranged in a parallel relationship, positioned in a single plane, and encapsulated with a flattened, oval-shaped jacket. To reduce the size of the insulated conductor, the knitted nylon covering over the served tinsel was eliminated and was replaced with a crystalline thermoplastic elastomer which is disclosed in U.S. Pat. No. 4,090,763 incorporated by reference hereinto and which serves as the primary retractile component in a spring-type telephone cord.
Telephone cords have high visibility coupled with high exposure to wear, staining and environmental degradation. Staining and discoloration are significant problems especially with equipment that receives heavy use and has a long service life. These problems have been overcome by a cord in which the jacket is coated as disclosed in U.S. Pat. No. 4,166,881 which was issued on Sept. 4, 1979 in the names of W. I. Congdon, J. J. Mottine and W. C. Vesperman. The coating has a stiffness modulus of about 1700 Kg/cm.sup.2, as measured in accordance with ASTM specification D-747, which is substantially greater than that, i.e. about 70 Kg/cm.sup.2, of the jacket material, but less than that, i.e. about 5300 Kg/cm.sup.2, of the conductor insulation.
It has been found that if top coated cordage is formed into a spring cord configuration, it has excellent retractile properties. However, when top-coated cordage is formed on mandrels of automatic cord making apparatus such as that shown in U.S. Pat. No. 3,988,092 which was issued on Oct. 26, 1976 in the names of G. F. Bloxham et al, the finished cords are so strongly retractile that excessive forces must be applied to them to distend their convolutions.
This problem occurs not only because of the top coating but also because of the relatively small diameter of the convolutions of the cordage. That diameter which is about 0.64 cm. could be increased by forming the convolutions on larger diameter mandrels to achieve a top-coated cord having a larger diameter such as for example on the order of 0.95 cm. Although such cords are suitably extensible, they suffer from lack of retractility. This is particularly noticeable in cords which are used on wall-mounted telephones and which are desired to have an extended length of 7.6 meters and a retracted length of about 1 meter.
In the prior art, it has been recognized that it is sometimes desirable to have differential retractility along the length of the cord. For example, see U.S. Pat. No. 2,704,782 which was issued on Mar. 22, 1955 in the name of W. L. Ames in which such cords are made by regulating the amount of axial twist which is caused to be imparted to cordage prior to or during its winding on a tapered mandrel or both. Because of the construction of the cordage in the above-mentioned patent, including its circular cross-section and because of the manner in which it is wound on a mandrel, the wound cordage always possessed an amount of axial twist. The axial twist was necessary because the materials used to insulate the conductors and to form the jacket did not provide sufficient retracility after the cordage was wound and heat-treated and after the direction of the helix was reversed.
While the cord in the above-identified Ames patent provided a measure of differential retractility, the twist varies along the cord and modulated the retractility and extensibility. Moreover, not only does the geometry of modular cords not lend itself to the use of axial twisting, but the plastic materials, i.e. of the insulation covering and of the coating, impart retractility to the cord thus obviating the need for axial twist. What is needed and what does not appear to be included in the prior art is a retractile cord of modulator construction which has a controlled extensibility as well as retractility to permit facile extension with assurance of convolution return to a compact helix.
The problem is to provide a cord which desirably may be made on existing capital equipment and which, while relatively long, has sufficient retractility to prevent sag. Moreover, it must be one which is sufficiently extensible so as not to require excessive forces to move a handset which is connected through the cord to a base. The sought-after cord should be one in which the extensibility and the retractility vary inversely linearly between the ends and the center of the cord. While the prior art includes top-coated cords and cords having varying retractility brought on by a combination of overtwist, axial twist and/or tapering, the art does not appear to provide a cord that meets the foregoing needs.